1. Field of the Invention
This invention relates to analog-to-digital converter circuitry and more particularly to dynamic range extender apparatus and methods in analog-to-digital converter circuitry for camera and imaging systems.
2. Description of Related Art
In recent years, solutions to difficult mixed signal problems related to dynamic range control in camera and imager devices and systems have been attempted. Particularly, it has been desired to develop low cost and low power approaches to improving the dynamic range of digital images. Further, new solutions to image data acquisition and processing have been attempted to result in visible improvements in image quality. Digital camera image quality improvement are sought for video as well as still image camera systems and imaging systems which use charge-coupled device (CCD) imagers, CMOS imagers, and other kinds of imagers.
It is known that the number of bits required for analog-to-digital conversion of CCD data depends upon the noise floor of a CCD, based upon photon shot noise, dark-current noise, and thermal noise from a CCD output amplifier. A system to capture the CCD output requires a quantization noise level lower than the noise floor. The maximum output of the CCD and the noise floor of the CCD can be used to determine the maximum number of bits required for an analog-to-digital converter to have its quantization noise level below the noise level of the CCD. For a particular CCD, the noise voltage level is estimated at about 150 .mu.Vrms. The maximum CCD output voltage is about 800 mV. Based upon these conditions, a 12-bit analog-to-digital converter is useful based upon dynamic range requirements. Unfortunately, a 12-bit converter is costly in terms of power and area.
It is further desirable to achieve enhanced image quality with images having improved detail in both dark and light image regions, while avoiding the penalties of high power consumption and large silicon area usage.
U.S. Pat. No. 4,647,975, entitled "Exposure Control System for an Electronic Imaging Camera Having Increased Dynamic Range" describes an electronic imaging system with an expanded dynamic exposure range implemented in two exposure intervals.